The Cambro‐Ordovician Cap Enragé Formation, Queébec, Canada: conglomeratic deposits of a braided submarine channel with terraces
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ABSTRACTThe Cambro‐Ordovician Cap Enragé Formation is interpreted as a deep submarine channel complex of conglomerates, pebbly sandstones and massive sandstones. The formation is up to 270 m thick, and crops out in a coastal belt 50 km long. In general terms, it has previously been interpreted as a deep sea channel deposit, with the channel about 300 m deep, at least 10 km wide and trending south‐westward, parallel to the coastal outcrops. Eight facies have been defined in this study and they have been grouped into three major facies associations. In the Coarse Channelled Association, conglomerates with carbonate boulders up to about 4 m are associated with graded‐stratified finer grained conglomerates. Facies of this association make up about 25% of all the beds in the formation. The association is also characterized by abundant major channels 1–10 m deep and up to 250 m wide. Excellent outcrop allows the reconstruction of topographic highs (bars) within the channels and the association is interpreted as a braided channel and bar system.The second association, Multiple‐Scoured Coarse Sandstones, contains some graded‐stratified fine conglomerates, along with massive to structureless coarse and pebbly sandstones, and rare cross‐bedded pebbly sandstones. Deep channels are absent, but multiple channelling on the scale of 0.5–1 m is characteristic. In the absence of the very coarse conglomerates and deeper channelling, this association is interpreted as being deposited on topographically higher terrace areas adjacent to the main braid plain.The third facies association, Unchannelled Sandstones, is characterized by massive sandstones with abundant fluid‐escape structures, classical turbidites and thin shales. In the absence of any scouring deeper than a few tens of centimetres, this association is interpreted as being deposited on an even higher and smoother terrace, farther from the braid plain.Palaeoflow directions for conglomerate facies indicate fairly consistent south‐westward transport, apparently parallel to the base of the Cambro‐Ordovician continental slope. Flow directions in the finer‐grained facies are rather variable, suggesting complex bar development and overbank spills.Thinning‐and fining‐upward sequences are present on two scales. The smaller, 1–10 m sequence, is related to channel filling and abandonment. Thicker sequences (10–100 m), with facies of the Multiple Scoured, and Unchannelled Sandstone Associations, may indicate switching of a main channel away from the area and its subsequent burial by marginal terrace and higher terrace deposits.
